Diarrhea can be a symptom of several types of cancer, but it is far more commonly caused by infections, food sensitivities, medications, or digestive conditions like irritable bowel syndrome. The cancers most likely to cause diarrhea are those that form in or near the digestive tract: colon cancer, rectal cancer, pancreatic cancer, neuroendocrine tumors, and lymphomas that start in the gut. What matters most is the pattern of your symptoms and whether they appear alongside other warning signs.
Which Cancers Cause Diarrhea
Colorectal cancer is the most well-known connection. A large study published in the JNCI found that diarrhea appearing 3 months to 2 years before a colorectal cancer diagnosis was about 1.4 times more common in people who went on to be diagnosed compared to those who didn’t. That’s a modest increase in risk, not a dramatic one, which underscores that most people with diarrhea do not have cancer. But when diarrhea shows up alongside rectal bleeding, abdominal pain, or iron deficiency anemia, the combination becomes more significant. Iron deficiency anemia carried roughly double the risk in that same study.
Pancreatic cancer causes diarrhea through a different mechanism. When a tumor blocks or damages the pancreas, the organ can no longer produce enough digestive enzymes to break down fat. The result is a condition called exocrine pancreatic insufficiency, which produces loose, greasy, foul-smelling stools along with excess gas. This type of diarrhea looks and feels different from a typical stomach bug. The stools may float, appear oily, or be unusually pale.
Neuroendocrine tumors, particularly those in the small intestine, are a less common but distinctive cause. These tumors can secrete hormones, especially serotonin, directly into the bloodstream. Normally only about 1% of a dietary building block called tryptophan gets converted to serotonin, but neuroendocrine tumors can divert up to 70% of it. That flood of serotonin speeds up gut movement and triggers the intestines to dump fluid, producing watery diarrhea. This is part of a broader pattern called carcinoid syndrome, and diarrhea affects about 80% of patients with it. Facial flushing and wheezing often accompany the diarrhea, which makes this presentation fairly recognizable.
Ovarian cancer is worth mentioning because it’s not in the digestive tract, yet about 20% of women diagnosed with it report a change in bowel habits beforehand. Ovarian tumors can press on nearby bowel structures or cause fluid buildup in the abdomen, leading to diarrhea, constipation, or alternating between the two.
How Cancer Causes Diarrhea
Your intestines work by balancing how much fluid they absorb and how much they secrete. Cancer disrupts this balance in several ways. A tumor growing in the colon or rectum can physically narrow the passage, irritate the intestinal lining, or cause inflammation that prevents normal water absorption. The result is loose, frequent stools, sometimes with blood or mucus.
Hormone-secreting tumors take a different route. The excess hormones they release, like serotonin or vasoactive intestinal peptide (VIP), directly stimulate the gut to push contents through faster and secrete more fluid than it absorbs. This type of diarrhea tends to be watery and high-volume, and it often persists even when you haven’t eaten.
Pancreatic tumors cause problems upstream. Without enough digestive enzymes reaching the small intestine, fat passes through undigested. The undigested fat draws water into the bowel, producing the characteristic greasy diarrhea.
Warning Signs That Deserve Attention
A single episode of diarrhea, or even a few days of loose stools after a questionable meal, is almost never cancer. The patterns that warrant medical evaluation are more specific:
- Duration: Diarrhea lasting more than four weeks is classified as chronic and should be investigated regardless of suspected cause.
- Blood in the stool: Rectal bleeding combined with a change in bowel habits is the strongest red flag for colorectal cancer.
- Unexplained weight loss: Losing weight without trying, especially alongside persistent diarrhea, raises concern for cancers of the digestive tract or pancreas.
- Iron deficiency anemia: Feeling unusually fatigued, pale, or short of breath can signal slow blood loss from a tumor in the colon, even when you don’t see blood in your stool.
- Greasy or oily stools: This pattern specifically points toward problems with fat digestion and warrants pancreatic evaluation.
- Flushing and wheezing with diarrhea: This combination suggests carcinoid syndrome from a neuroendocrine tumor.
Age matters too. Colorectal cancer rates have been rising in adults under 50, and the study from JNCI specifically examined early-onset cases. Abdominal pain, rectal bleeding, diarrhea, and iron deficiency anemia were all predictive signs appearing months to years before diagnosis in younger adults.
How Doctors Rule Out Cancer
If your doctor suspects that chronic diarrhea could be related to cancer, the evaluation typically follows a stepwise approach. The first tool is often a stool test called a fecal immunochemical test (FIT), which detects tiny amounts of blood invisible to the naked eye. This test has a negative predictive value of about 99%, meaning that if it comes back clean, it’s very unlikely you have colorectal cancer. It’s a simple, noninvasive way to determine whether more investigation is needed.
When the FIT test is positive, or when symptoms are concerning enough on their own, a colonoscopy is the standard next step. British Society of Gastroenterology guidelines recommend colonoscopy with biopsy as the preferred investigation for chronic diarrhea. During the procedure, a doctor can directly visualize the colon and take tissue samples from anything that looks abnormal. For suspected pancreatic cancer, imaging like a CT scan or MRI is typically the first move, sometimes paired with blood tests for specific tumor markers.
Neuroendocrine tumors are diagnosed through a combination of blood and urine tests that measure serotonin and its byproducts, followed by specialized imaging to locate the tumor.
Diarrhea From Cancer Treatment
It’s worth distinguishing diarrhea caused by cancer itself from diarrhea caused by cancer treatment, since the two are very different situations. Chemotherapy is one of the most common causes of diarrhea in cancer patients. These drugs damage the fast-dividing cells lining the intestines, leading to inflammation, reduced absorption, and excess fluid secretion. The lining of the small bowel can develop something like superficial tissue damage, disrupting the delicate balance between absorbing nutrients and secreting digestive fluids.
Certain chemotherapy drugs are particularly notorious for causing severe diarrhea. The mechanism involves direct damage to the intestinal lining combined with increased mucus production and changes in how quickly food moves through the gut. Radiation therapy targeting the abdomen or pelvis can cause similar problems. If you’re already undergoing cancer treatment and develop new or worsening diarrhea, your oncology team can adjust medications or add supportive treatments to manage it.
The key distinction: diarrhea that appears as a new, unexplained symptom before any cancer diagnosis is the scenario where evaluation matters most. Diarrhea during treatment is expected, manageable, and closely monitored by your care team.